Chemical attachment of the hydrophilic polymer poly(ethylene glycol) (“PEG”) to molecules and surfaces is of great utility in biotechnology. In its most common form PEG is a linear polymer terminated at each end with hydroxyl groups:HO—CH2CH2O—(CH2CH2O)n—CH2CH2—OHThis polymer can be represented in brief form as HO-PEG-OH where it is understood that the -PEG- symbol represents the following structural unit:—CH2CH2O—(CH2CH2O)n—CH2CH2—In typical form n ranges from about 10 to about 2000.
PEG is commonly used as methoxy PEG-OH, or mPEG in brief, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification.CH3O—(CH2CH2O)n—CH2CH2—OH mPEG
PEG is also commonly used in branched forms that can be prepared by addition of ethylene oxide to various polyols, such as glycerol, pentaerythritol and sorbitol. For example, the four-arm, branched PEG prepared from pentaerythritol is shown below:C(CH2—OH)4+nC2H4O→C[CH2—O—(CH2CH2O)n—CH2—CH2—OH]4 
The branched PEGs can be represented in general form as R(-PEG-OH)n in which R represents the central “core” molecule, such as glycerol or pentaerythritol, and n represents the number of arms.
Branched PEGs can also be prepared in which two PEG “arms” are attached to a central linking moiety having a single functional group capable of joining to other molecules: e.g., Matsushima et al., (Chem. Lett., 773, 1980) have coupled two PEGs to a central cyanuric chloride moiety.
PEG is a well known polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule “conjugate” soluble. For example, it has been shown that the water-insoluble drug paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331 336 (1995).
In related work, U.S. Pat. No. 4,179,337 to Davis et al. discloses that proteins coupled to PEG have enhanced blood circulation lifetime because of reduced rate of kidney clearance and reduced immunogenicity. These and other applications are also described in Biomedical and Biotechnical Applications of Polyethylene Glycol Chemistry, J. M. Harris, Ed., Plenum, N.Y. (1992), and Poly(ethylene glycol) Chemistry and Biological Applications, J. M. Harris and S. Zalipsky, Eds., ACS, Washington D.C. (1997).
To couple PEG to a molecule such as a protein, it is often necessary to “activate” the PEG to prepare a derivative of the PEG having a functional group at the terminus. The functional group can react with certain moieties on the protein such as an amino group, thus forming a PEG-protein conjugate. Many activated derivatives of PEG have been described. An example of such an activated derivative is the succinimidyl succinate “active ester”:
Hereinafter, the succinimidyl active ester moiety will be represented as —CO2—NS in chemical drawings.
The succinimidyl active ester is a useful compound because it reacts rapidly with amino groups on proteins and other molecules to form an amide linkage (—CO—NH—). For example, U.S. Pat. No. 4,179,337 to Davis et al. describes coupling of this derivative to proteins (represented as PRO—NH2):mPEG-O2CCH2CH2CO2NS+PRO—NH2→mPEG-O2C—CH2CH2—CONH—PRO
Bifunctional PEGs with active groups at both ends of the linear polymer chain are also useful compounds when formation of a crosslinked insoluble network is desired. Many such bifunctional PEGs are known in the art. For example, U.S. Pat. No. 5,162,430 to Rhee, et al. discloses using such bifunctional PEGs to crosslink collagen.
Reactive PEGs have also been synthesized in which several active functional groups are placed along the backbone of the polymer. For example, lysine-PEG conjugates have been prepared in the art in which a number of activated groups are placed along the backbone of the polymer. Zalipsky et al. Bioconjugate Chemistry, 4:54 62 (1993).
U.S. Pat. No. 5,283,339 to Arnold et al. discloses PEG compounds capable of chelating metals. The PEG compounds have a terminal metal chelating group which has two free carboxylic acid or amino groups, typically linked to a nitrogen atom. The PEG compounds are used to extract and precipitate proteins from solutions with the carboxylic acid or amino groups together with the nitrogen atom capable of forming ionic complexes with metal ions. However, the metal chelating groups disclosed in the patent generally are not useful in covalently coupling the PEG compounds to proteins, peptides, or small drugs bearing functional groups such as amines. The patent does not teach forming an activated PEG derivative for covalently coupling to another molecule to form a conjugate.